output.txt

taskid=5 creating new list
taskid=0 creating new list
taskid=1 creating new list
taskid=2 creating new list
taskid=3 creating new list
taskid=4 creating new list
taskid=6 creating new list
taskid=7 creating new list
taskid=8 currentValue=1
taskid=0 currentValue=2
taskid=2 currentValue=3
taskid=1 currentValue=4
taskid=3 currentValue=5
taskid=4 currentValue=6
taskid=6 currentValue=7
taskid=7 currentValue=8
taskid=9 currentValue=9

source.cs

using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace ConsoleApp2
{
    class Program
    {

        private static object _lock = new object();
        
        static void Main(string[] args)
        {
            var dic = new ConcurrentDictionary<string, List<int>>();
            var allTasks = new Task[10];
            for (int i = 0; i < 10; i++)
            {
                int taskId = i;   
                allTasks[i] = (Task.Run(() =>
                {
                    dic.AddOrUpdate("0", s =>
                        {
                            Console.WriteLine($"taskid={taskId} creating new list");
                            return new List<int> { 0 };
                        },
                        (s, ints) =>
                        {
                            lock (_lock)
                            {
                                var max = ints.Max();
                                var newMaxValue = max + 1;
                                ints.Add(newMaxValue);
                                Console.WriteLine($"taskid={taskId} currentValue={newMaxValue}");
                                return ints;
                            }
                        });
                }));
            }

            Task.WaitAll(allTasks, Int32.MaxValue);
            Console.ReadLine();
        }
    }
}